It is known that hydrocarbons may be recovered by the destructive heating of such formations as oil shale and coal deposits. The recovery of valuable hydrocarbon products from such sources, particularly certain sedimentary rocks which are commonly referred to as oil shale, has been sought for many years and numerous processes have been developed whereby relatively crude oil, as well as gaseous hydrocarbons, may be produced therefrom. Extensive deposits of oil shale are to be found in this country particularly in the so called Green River Shale formation located in the states of Colorado, Utah, and Wyoming. Important shale deposits are also found in other parts of the world. With diminishing world petroleum reserves and accelerating world demand for petroleum products there exists a necessity for developing a commercially feasible process, suitable for application on a large scale for retorting (i.e. destructive distilling) oil shale to recover its potential yield of crude oil.
The two principal engineering problems connected with oil-shale retorting on a large scale are those of materials handling and of heat exchange. In such an operation literally thousands of tons of shale must be moved through the retorting vessel and auxiliary heat exchange vessel if any. This shale must be heated in some manner to retorting temperatures of the order of 800.degree. to 1000.degree. Fahrenheit. This involves the exchange of enormous quantities of heat since not only the organic matter (usually termed kerogen) must be heated, but also the inert, inorganic portion of the shale, which usually comprises from 80% to 90% by weight of the total shale.
With respect to the materials handling problem, obviously the simplest and most inexpensive manner of operation would be to feed the shale downwardly by gravity through the retorting vessel.
Thus far the most attractive manner of heating the shale to retorting temperature appears to be direct heat exchange between the shale as a bed of broken solids, and a hot gas stream flowing through the shale bed. In a continuous process, the shale bed and gas stream preferably flow counter-currently to one another. In such a process it is highly desirable, from the standpoint of thermal efficiency, and from the standpoint of certain operational difficulties, otherwise encountered, that the outgoing bed of shale and the outgoing gas stream, containing the product oil, both leave the processing vessel at low temperature.
It is particularly desirable that the product gas stream, carrying the oil, give up most of its heat to the incoming shale before leaving the retort. First of all, at high gas exit temperature (e.g. 400.degree. to 500.degree. F) there is a strong tendency for the oil vapors to deposit coke upon the walls of the outlet ducts, eventually plugging the outlets completely, thus causing periodic shutdowns. In addition to the coking problem, a high exit temperature for the oil-bearing gas stream requires the use of expensive cooling equipment to condense out the oil vapors from the relatively large volume of retorting gases. Furthermore large quantities of cooling water would also be required. In this country, where substantially all the commercially interesting oil-shale deposits are located in arid regions, this type of operation would be practically out of the question in a large scale operation.
Likewise it is important that the retorted shale be discharged at a low temperature (200.degree. to 250.degree. F). Higher temperatures present disposal problems.
It is also very important that there be no metallic parts exposed to combustion zone temperatures. It is also important that the yield of shale oil approach the optimum because retorting costs are nominal compared to the cost of mining the oil-shale and of disposing of the spent shale.
In an effort to achieve the ends listed above, literally hundreds of retorting processes have been proposed, each of which offers a somewhat different choice and combination of the many possible operating conditions. Many of these prior processes possess in a fairly high degree one or more of the above desiderata but most, in one important respect at least, seriously fail to fulfill the requirements of an ideal retorting process.